Radio access networks (RANs) of different wireless cellular communication systems operate according to various different defined sets of standards each having different respective radio access technologies (RAT). Wireless terminals operated by users of such systems are usually configured so that they can operate with more than one such radio access network. Therefore it is often required that a terminal must be able to connect to more than one type of network operating according to different respective standards, and must be able to switch dynamically between such different networks. Such a switching process is often termed an inter-RAT ‘handover’.
GSM (Global System for Mobile Communications, originally Groupe Spécial Mobile), is a set of standards developed by the European Telecommunications Standards Institute (ETSI) to define technologies for so-called second generation (2G) digital cellular networks. Developed as a replacement for first generation (1G) analog cellular networks, the GSM standard originally described a digital, circuit switched network optimized for full duplex voice telephony. The standard was expanded over time to include first circuit switched data transport, then packet data transport via GPRS (General Packet Radio Services).
Enhanced Data rates for GSM Evolution (EDGE) (also known as Enhanced GPRS (EGPRS)) is a digital mobile telephone technology that is a backward-compatible extension of GSM and allows improved data transmission rates. EDGE is considered a pre-third generation (3G) radio technology and was deployed on GSM networks beginning in 2003. EDGE is standardized by the international organisation known as 3GPP (3rd Generation Partnership Project) as part of the GSM family. A network that operates according to the GSM and/or EDGE standards is known as a GSM/EDGE radio access network (GERAN).
Universal Mobile Telecommunications System (UMTS) is a third generation mobile cellular technology for communication networks based on the GSM standard. Developed by 3GPP, UMTS is a component of a set of standards specified and maintained by the International Telecommunications Union (ITU), this set of standards known as IMT-2000. IMT-2000 is comparable to, but different from, the CDMA2000 set of standards for networks based on the competing cdmaOne™ technology deployed in the USA and internationally elsewhere.
UMTS employs Wideband Code Division Multiple Access (W-CDMA) radio access technology to offer greater spectral efficiency and bandwidth. UMTS specifies a complete network system, covering the radio access network (UMTS Terrestrial Radio Access Network, or UTRAN), the core network (Mobile Application Part, or MAP) and the authentication of users via SIM cards (Subscriber Identity Module).
UMTS and GSM/EDGE can share a Core Network (CN), making UTRAN an alternative radio access network to GERAN, and allowing (mostly) transparent switching between these radio access networks (RANs) according to available coverage and service needs. Because of that, UMTS and GSM/EDGE radio access networks are sometimes collectively referred to as UTRAN/GERAN. Most cells of European mobile cellular communication systems, and most handsets or mobile terminals used in such networks, can support both UTRAN and GERAN operation.
Since 2006, UMTS networks in many countries have been or are in the process of being upgraded with High Speed Downlink Packet Access (HSDPA), sometimes known as 3.5G. Currently, HSDPA enables downlink transfer speeds of up to 21 Mbit/s. Work is also progressing on improving the uplink transfer speed with the High-Speed Uplink Packet Access (HSUPA). Longer term, the 3GPP Long Term Evolution (LTE™) set of standards, termed fourth generation (4G), aims to provide data transfer rates of 100 Mbit/s on the downlink and 50 Mbit/s on the uplink, using a 4G air interface technology based upon orthogonal frequency-division multiplexing (OFDM).
E-UTRAN is an abbreviation for evolved UMTS Terrestrial Radio Access Network.
E-UTRAN or eUTRAN is the radio access network defined by the LTE™ standards.
E-UTRAN uses a simplified single node architecture consisting of eNBs (E-UTRAN Node B)—see FIG. 1.
Referring to FIGS. 1 and 2, an eNB 102 communicates with an Evolved Packet Core (EPC) 202 using an S1 interface 104. Specifically the eNB 102 communicates with a MME (Mobility Management Entity) node 106, 206 and a UPE (User Plane Entity) node identified here as a S-GW (Serving Gateway) 108, 208 using S1-C and S1-U interfaces 104 for control plane and user plane respectively. The MME node 106, 206 and the UPE node 108, 208 are preferably implemented as separate network nodes so as to allow independent scaling of the control and user plane. Also any eNB can communicate with other eNBs using an X2 interface (X2-C and X2-U for control and user plane respectively). eNBs transmit signals to, and receive signals from, wireless terminals or ‘user equipments (UEs). Thus a wireless terminal can be connected to the MME via a eNB.
A HSS (Home Subscriber Server) (not shown) is a central database that contains user-related and subscription-related information. Functions of the HSS include mobility management, call and session establishment support, user authentication and access authorization.
The MME (Mobility Management Entity) is the key control-node for the E-UTRAN access network. It is responsible for idle mode UE (User Equipment) tracking and paging procedures including retransmissions. It is involved in bearer activation/deactivation processes and is also responsible for choosing the serving gateway (S-GW) for a UE when the UE initially attaches to the network and during intra-E-UTRAN handover involving Core Network (CN) node relocation. It is responsible for authenticating the user (by interacting with the HSS).
Non Access Stratum (NAS) signalling terminates at the MME and the MME is responsible for generation and allocation of temporary identities to UEs. The MME checks the authorization of the UE to register with the service provider's Public Land Mobile Network (PLMN) and enforces UE roaming restrictions. The MME also provides the control plane function for mobility between LTE and UTRAN/GERAN access networks with the S3 interface terminating at the MME from the SGSN of the UTRAN/GERAN network.
The Serving GPRS Support Node (SGSN) of a UTRAN/GERAN network has a similar overall function to that of the MME of the E-UTRAN network. The SGSN is responsible for the delivery of data packets from and to the wireless terminals (‘mobile stations’) within its geographical service area. Its tasks include packet routing and transfer, mobility management (attach/detach and location management), logical link management, and authentication and charging functions. The location register of the SGSN stores location information (e.g., current cell, current visitor location register, VLR) and user profiles (e.g., IMSI, address(es) used in the packet data network) of all GPRS users registered with the SGSN.
Among other functions, the SGSN performs functions associated with mobility management required when a wireless terminal in standby mode moves from one Routing Area (RA) to another Routing Area.
E-UTRAN specifies an Idle mode Signaling Reduction (ISR) function which provides a mechanism to limit or reduce signaling in idle mode during any inter-RAT cell-reselection between E-UTRAN and UTRAN/GERAN. According to this mechanism a wireless terminal (User Equipment, UE) in idle mode, when ISR is activated, is registered with both the MME of the E-UTRAN and the SGSN of a UTRAN/GERAN (see 3GPP TS23.401, Annex J1). Both the SGSN and the MME have a control connection with the serving gateway (S-GW). The MME and SGSN are both registered at the HSS. The UE receives and stores mobility management (MM) parameters provided to the UE by the SGSN (e.g. P-TMSI and RA) and provided to the UE by the MME (e.g. GUTI and TA(s)) and the UE stores session management (bearer) contexts that are common to E-UTRAN and GERAN/UTRAN accesses.
Using these stored parameters and contexts, the UE when it is in idle state can reselect between E-UTRAN and GERAN/UTRAN radio access cells when the UE is within the registered radio access routing areas (RAs) and tracking areas (TAs) without any need to perform any tracking area update (TAU) or radio access update (RAU) procedures by interacting with the network.